1. Field of the Invention
This invention relates to an electroless plating bath for forming a coating of an alloy of nickel and phosphorus on a metal or nonmetal surface.
2. Description of the Prior Art
It is known that an electroless plating bath containing hypophosphite as a reducing agent can be used to form a coating of an alloy of nickel and phosphorus, as disclosed in, for example, KINZOKU HYOMEN GIJUTSU (Metal Surface Treating Techniques), vol. 32, No. 12 (1981), pages 600 to 609. The electroless plating baths are classified into acidic and alkaline ones. The alkaline baths are classified into caustically and ammoniacally alkaline ones. The nickel-phosphorus alloy coatings which these baths can form, however, have a low phosphorus content as expressed by atomic percentage relative to nickel. The coating formed by an ammoniacally alkaline bath has a phosphorus content of only 3 to 5%. The coating formed by an acidic bath has a phosphorus content of only 8 to 10%. Although a caustically alkaline bath can form a coating having a somewhat higher phosphorus content (dozens of percent) than an acidic bath, it is practically unusable, since it has a very low deposition rate, which is less than a half of that of an acidic bath, and moreover lacks stability.
It is known that a coating of an alloy of nickel and phosphorus having a phosphorus content of only 3 to 5% has a face-centered cubic lattice structure, and that a coating having a phosphorus content of about 7.4% or above has an amorphous structure.
It is also known from the results of research on an amorphous alloy made by, for example, the fast cooling of a liquid metal that the structure and properties of a two-component amorphous alloy composed of a metal and a semi-metal depend largely on the semimetal, or in the instant case, phosphorus content thereof. Researchers in various fields of active research on amorphous alloys are predominantly of the opinion that the majority of amorphous alloys of nickel and phosphorus having practically aceeptable properties have a phosphorus content of 15 to 25 atom %. The alloy coating formed by electroless plating is not an exception. The inventor of this invention has made extensive research including the quantitative analysis of nickel and phosphorus in their alloys by EPMA and the X-ray diffraction thereof, and found that the coatings having a phosphorus content of 7 to 15 atom % have an amorphous structure, but are not alloys which are practically acceptable for use under severe conditions from the standpoints of resistance to corrosion and wear, deformability and nonmagnetic property. A plating bath containing hypophosphite as a reducing agent is more often used to form a coating of an alloy of nickel and phosphorus than any other type of bath. In order to increase the phosphorus content of the coating, it is generally believed effective to increase the hypophosphite content of the bath and lower its pH. These expedients, however, have fatal defects. The excessive addition of hypophosphite brings about the excessive accumulation of phosphorous acid ions and the resulting formation of nickel phosphite, thereby accelerating the decomposition of the bath. The lowering of the pH of the bath brings about a drastic reduction in its deposition capacity.